1. Field of the Invention
This invention relates to a blow-molded bottle-shaped container of biaxially oriented polyethylene terephthalate resin and, more particularly, to a bottle-shaped container in which large durable strength is created against an increase in the pressure in the bottle-shaped container but which is easily and uniformly deformed under reduced pressure in the container.
2. Related Art
It is known that a blow-molded bottle-shaped container of biaxially oriented polyethylene terephthalate resin (hereinafter referred to as "PET") achieves improved heat resistance by heat setting the resin after biaxial-orientation blow-molding to provide a heat resistant bottle-shaped container for liquid to be filled into the container at high temperature, such as juice drink.
However, the bottle-shaped container of PET of this type does not have high rigidity like a glass or metal bottle-shaped container but is flexible. Thus, the body of the bottle-shaped container is improperly deformed under reduced pressure generated in the container due to volumetric contraction of the liquid or a decrease in the vapor pressure of a head space when filling the liquid at high temperature to cause the container to be remarkably defected in its external appearance.
The bottle-shaped container of PET of this type is prevented from being deformed in the configuration of the body by recessing and aligning flat longitudinal reduced pressure absorbing panels on its body to absorb the reduced pressure in the container by means of the panels.
Pressure and stress act on the panels of the heat resistant bottle-shaped container of (1) PET as described below. Hydraulic pressure produced due to the difference in height of the surface of the liquid in the container from the liquid in a tank when pressing to seal the neck of the container and (2) filling the liquid into the container by a filling machine with liquid at high temperature acts on the panels of the container. The hydraulic pressure equilibrates with the atmospheric pressure after filling the content liquid in the container. Internal pressure in the container increases due to vapor pressure in the head space of the container at the time of capping the neck of the container (e.g., the internal pressure in the container is raised to approx. 1.7149 kg/cm.sup.2 when the content liquid of 90.degree. C. is, for example, filled in the container). The vapor pressure in the container is reduced gradually from the time of capping to atmospheric pressure at the time of sterilization, and the pressure in the container is decreased in response to the pressure change caused by the liquid being reduced in volume when cooled and by the reduction in the vapor pressure in the head space of the container. The deforming stresses are generated at the panels in response to the pressure change.
As described above, the panels are affected by the heat from liquid in the container and also subjected to pressure changes when pressurizing (at the time of filling the container or capping the neck of the container), to the ambient pressure (immediately after filling the container) or to pressure reduction (when cooling the container). Therefore, the panels are heated to high temperature and pressurized to high pressure when filling the container, and capping the neck of the container, due to the vapor pressure and the heat of the liquid immediately thereafter, and are deformed so as to exhibit a raised shape at the outside of the container as compared with an empty container.
According to a number of experiments, generated vapor pressure is relatively low when the temperature of the liquid to be filled is 80.degree. C. or lower, so that the effects of temperature on the container are reduced. Thus, the stress to which the container can be additionally subjected is large, so that the extent to which the panels are deformed in a raised shape is relatively small, and the influence of the raised deformation of the panel, after cooling the container is very small. However, when the temperature of the content liquid is 85.degree. C. or higher and particularly 90.degree. C. or higher, generated vapor pressure in the container is larger, and the raised deformation of the panel after capping the neck of the container is much larger.
Since the raised deformation of the panel of the container is affected by the influence of the temperature of the content liquid and the vapor pressure of the container, a permanent strain remains in the material of the container due to a decrease in the strength of the material and the remaining strain.
The panels provided on the bottle-shaped container of this type are heretofore composed, in order to obtain uniform deformation, of (1) flat surfaces as large as possible on the entire area of the panels, (2) external projections of the entire panel in advance, (3) external protrusion of part of the panel in advance, (4) inclined surfaces of the panels to reduce the raised deformation, (5) recessed grooves surrounding on the panels to scarcely cause the panels to be deformed in a raised shape, and (6) lateral and longitudinal rib strips formed on the panels. However, when the temperature of the content liquid filled in the container is actually raised to 85.degree. C. or higher, raised deformations indispensably generated on the panels are increased due to the influence of the heat and vapor pressure of the liquid content in the container, and permanent deformation remains at the panel as remaining strains upon cooling the container. The panels which have once been subjected to the raised permanent deformation cannot function as ordinary panels and lose their reduced pressure absorbing action. Thus, the entire body of the container is improperly deformed to triangular or elliptical shape, or the panels cannot absorb the normal pressure reduction, thereby causing the external appearance of the container to be deteriorated.
As described above, it is also known that panels which cause less raised deformation against an increased pressure at the time of capping the neck of the container and also cause easy deformation due to recessed deformation under reduced pressure in the container at the time of cooling the container are formed in flat structure in the whole inside of the stepped portion of the panels surrounded by bent stepped portions on the periphery. However, mere flat structure of the entire panel causes the stepped portions to be subjected to permanent deformations as will be described so that the panels cannot absorb deformations due to normal reduced pressure. Even if the panels may absorb the reduced pressure deformation, the available state of the stress acting on the panels due to the reduced pressure cannot be specified to be uniform. Thus, predetermined stable deformation cannot be obtained at the panels. In this manner, the degrees of absorbing the deformation due to reduced pressure in the panels differ, so that the external appearance of the bottle-shaped container is abnormally deteriorated.
The most simple means which do not retain permanent deformation in the raised strains of the panels is to increase the heat setting effect of the container. The heat setting includes biaxial-orientation blow-molding a preformed piece by injection molding, then cooling the piece, then heating again the piece to remove its remaining stress, and thereafter further blowing the piece to complete a product. However, in order to raise the heat setting effect of the bottle-shaped container, it is necessary to raise the heat setting temperature and to increase the setting time. Thus, the heat setting remarkably reduces the productivity. Therefore, a method of raising the heat setting is not practical. Even if the container is sufficiently heat set in this manner, the deformation for the reduced pressure absorbing effects of the panels cannot be always uniformly generated, and adverse effects on the appearance of the container due to irregular deformation still remain unsolved.
Since blow-molded bottle-shaped containers of biaxially oriented synthetic resin are removed from a metal mold in a state in which the container is yet soft after blow-molding, the container may be deformed due to small remaining distortion. This distortion of the container is understood to be largely affected by the structure of the panels. The bottle-shaped container having conventional panels as described above has remarkable drawbacks in that its structure is readily deformed after blow-molding.
The causes of permanent deformation of the panel in the bottle-shaped container have been observed in detail. It is discovered that one of the causes resides in the fact that the bending angles of two bent portions of the stepped portions bent at the periphery of the panels are varied in directions opposite to each other to be different from the angle at the time of molding.
The variations in the bending angles of the two bent parts of the stepped portions was understood from the fact that permanent deformation occurred due to excessive deformation in opposite directions at the two bent parts due to the temperature and the vapor pressure of the liquid with which the container is filled. When the stepped portions are thus deformed, the entire panels remain deformed in raised shape, to resulting in impossibility of smoothly recessed distortion for absorbing reduced pressure in the container.
In a cylindrical bottle-shaped container, the body is located at an equal distance from the center line at any portion. Thus, the container is easily uniformly oriented. However, in a polygonal bottle-shaped container, the body is not located at equal distances from the center line; according to the positions, the container is subjected to irregular orientations. Therefore, the amounts of orientation are different at different positions on the container. Thus, internal remaining stresses generated by blow-molding are different at different positions on the body. The differences in the blow-molding cause the panels to be subjected to permanent deformations at the time of heat setting or completing the container. This is also remarkable particularly at the bottom of the container at the portions which are most feasibly affected by the orientation.